Car-on-track systems have been extensively used in the automobile industry as well as in other industries wherein materials must be transported from point to point. Additionally, car-on-track systems are used in assembly operations wherein a component supported on a carrier movable along the track may stop at one or more stations for assembly of parts to the component.
The basics of a car-on-track system are shown in the Gutekunst et al. U.S. Pat. No. 4,593,623, owned by the assignee of this application. The patent shows a carrier having rollers which movably support the carrier on a pair of spaced-apart rails. Movement of the carrier is derived from a rotatable drive tube extending along the track and which coacts with a drive wheel depending from the carrier and which is spring-loaded into engagement with the drive tube. As well known in this art, linear movement of the carrier is determined by the angle of the drive wheel relative to the rotatable drive tube. When the axis of rotation of the drive wheel is parallel to the axis of rotation of the drive tube, there is no linear movement of the carrier and there can be movement in either forward or reverse direction depending upon the canting of the drive wheel relative to the drive tube.
The Gutekunst et al. patent also shows accumulation means whereby a carrier will be caused to stop if, in its movement, a rod of the accumulation means contacts a preceding carrier.
A particularly unique utilization of basic concepts utilized in a car-on-track system, such as disclosed in the Gutekunst et al. patent referred to above, is shown in the Gutekunst et al. U.S. Pat. No. 4,648,325, owned by the assignee of this application. The last-mentioned patent shows a linear drive unit having a carriage with a plurality of drive wheels engageable with a drive tube, with the carriage being usable as a substitute for a hoist or other track-mounted conveying element. This patent shows the use of acceleration and deceleration cams for controlling the canting of the drive wheels relative to the drive tube as the carriage moves between a full-speed and stopped conditions.
In the car-on-track system disclosed in the first-mentioned Gutekunst et al. patent, the force of the spring acting to urge the drive wheel against the drive tube is reacted to the carrier and, thus, the carrier and the load supported thereby must have sufficient weight to hold the carrier onto the track rails against the force of the spring. This dictates that the carrier must have substantial weight, if the load is to be a light load, with such a carrier being more bulky and expensive because of its size and, therefore, increasing the cost of the system which will utilize many carriers.
There are also various known mechanisms for transferring a car or carrier between tracks having adjacent spaced-apart ends, with such structures including a turntable, as shown in the Jones U.S. Pat. No. 4,059,053; a shuttle car, as shown in Ziegenfus et al. U.S. Pat. No. 4,132,174; and a pivotally-mounted oscillatable member, as shown in Berk et al. U.S. Pat. No. 4,389,941. All of these patents disclose some form of drive tube on the transfer device for advancing a carriage or car onto the device and for discharging the carriage or car off the device and with the drive tube either deriving rotation from the drive tube of a track or from a motor mounted on the transfer device. An improvement disclosed in this application relates to the use of transfer devices which do not require any form of drive tube on the transfer device and which rely on carrier drive wheel contact with extensions of the drive tubes for moving the carrier onto the transfer device and for discharge of the carrier from the transfer device. Additionally, this feature is uniquely used in an over and under transfer system wherein there are a pair of superimposed tracks and a transfer device pivots about a horizontal axis to transfer a carrier from the upper track to the lower track for continuous forward travel of the carrier in returning to a starting point.